The invention relates to a method for controlling a receiver realized with the rake principle and comprising a number of correlator branches that can be synchronized with a received signal.
A receiver operating with the rake principle comprises several branches each of which may be synchronized with a different signal component. Therefore the receiver can receive several signals simultaneously. Rake receivers are used especially in CDMA receivers.
CDMA is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, for example the simplicity of frequency planning and spectral efficiency.
In the CDMA method, the narrow-band data signal of the user is multiplied to a relatively wide band by a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection between a base station and a mobile station, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of each user. The spreading codes are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other.
Correlators provided in conventional CDMA receivers are synchronized with a desired signal, which is recognized on the basis of a spreading code. The data signal is restored in the receiver to the original band by multiplying it again by the same spreading code as during the transmission. Signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band. They appear thus as noise with respect to the desired signal. The object is to detect the signal of the desired user from among several interfering signals. In practice, the spreading codes are not orthogonal and the signals of other users hinder the detection of the desired signal by distorting the received signal non-linearly. This interference caused mutually by the users is called multiple access interference.
For the performance of the spread spectrum system, it is of primary importance that the receiver can be synchronized with the incoming signal rapidly and accurately. The synchronization with the incoming signal usually takes place in two stages. In the acquisition of the code phase, the desired signal is to be located from the received transmission and the phase of the signal is to be determined with the accuracy of half a chip. When this has been completed, the phase is considered locked, whereafter the code phase is fine-tuned with a code tracking loop that is in charge of maintaining the phase locking.
The acquisition of the code phase can be implemented by means of either a matched filter or active correlation. The former method is fast but it is only applicable for short codes and it consumes a great deal of current when implemented digitally. Active correlation is the most often used method in CDMA systems. In active correlation, code phases of a local correlator are scanned in steps of half a chip and each phase is compared to the received signal. This implementation is economically advantageous but slow. The acquisition can be hastened by using several correlators in parallel so that the area of acquisition can be divided into several parts. The time of acquisition is then naturally shorter.
In prior arrangements the rake receivers have been designed in such a way that the acquisition of the new code phases takes place in a centralized manner in a separate searcher branch. The searcher branch searches for signals intended for the receiver and separate correlators are reserved for monitoring and demodulating the signals that have been found. Another known arrangement is to use all correlators for the acquisition when connecting to the system, and when the desired signal has been found, the operation of the correlators is invariable, i.e. one or two branches operate as searchers and the others monitor the desired signal.
The control algorithm for the branches of the rake receiver is a complicated sorting algorithm that is complex to implement as regards both software and hardware. Due to the complicated nature, calculating the control algorithm of the branches also takes up a great deal of time, which deteriorates the performance of the receiver.
The purpose of the present invention is to realize a receiver where the rake branches are controlled in a simple manner and the control is faster than in the conventional arrangements.
This is achieved with a method of the type described in the preamble, characterized in that each correlator branch in the receiver searches for advantageous signal components, synchronizes itself with a signal component it has found, and that each branch monitors and receives the signal component it has found.
The invention also relates to a receiver realized with the rake principle and comprising a number of correlator branches that can be synchronized with the received signal. The receiver according to the invention is characterized in that the correlator branches of the receiver comprise means for searching for advantageous signal components and means for guiding a branch to receive an advantageous signal component it has found.
In the method according to the invention, each branch in a rake receiver searches for a signal that it starts to receive and monitor. The branches of the rake receiver start operating with different delays in order to ensure the starting of fast reception. When the branches scan the different signal delays, the levels of the signals that have been found are compared either to a predetermined or an adaptive threshold value. This ensures that only a signal that is of sufficiently high quality is selected for reception. The arrangement according to the invention does not require a separate searcher block or a block calculating the allocation of the branches. The calculation of the allocation is especially complicated, since it requires a considerable amount of software and hardware, and takes a relatively long amount of time.